1. Field of the Invention
The invention relates to a diagnostic system for an automatic transmission, which diagnoses an automatic transmission installed in a drive train of a vehicle to determine the presence of faults therein.
2. Description of the Related Art
As well known in the art, an automatic transmission installed in a drive train of the vehicle includes a plurality of friction devices, such as clutches and brakes, to which hydraulic pressure is selectively applied so as to engage the friction devices. With the selected friction device(s) thus engaged, a power transmission path, or a gear position of the transmission, is determined depending upon the operating conditions of the vehicle. More specifically, a hydraulic pressure source is provided which is driven through rotation of an output shaft of an internal combustion engine so as to supply hydraulic oil to each of the friction devices, and a plurality of solenoid valves are placed in conjunction with the respective friction devices. A control system, which performs various controls of the automatic transmission in integrated manners, controls the degrees of opening of the solenoid valves by controlling the amounts of current passed through the solenoid valves. In the automatic transmission of this type, a target gear position is set based on the operating conditions of the vehicle, and a selected one or ones of the friction devices is/are engaged while a selected one or ones of the friction devices is/are released so as to shift the transmission to the thus set target gear position. To engage each of the selected friction devices, the control system causes a required amount of current to be fed to the solenoid valve associated with the friction device to be engaged, so as to increase the opening of the solenoid valve and thus increase the hydraulic pressure applied to the friction device. To release each of the selected friction devices, the control system operates to de-energize the solenoid valve associated with the friction device to be released, so as to close the solenoid valve and reduce the hydraulic pressure applied to the friction device.
In the automatic transmission as described above, if a fault, such as disconnection or a short-circuit, occurs to a solenoid of any of the solenoid valves, the transmission may suffer a problem that, for example, a friction device to be engaged for shifting is not able to be engaged, or a problem that a friction device to be released for shifting is not able to be released. In this case, the automatic transmission may not be able to be shifted to the target gear position, or the automatic transmission may lock in itself, which naturally results in deterioration of the shifting performance of the automatic transmission.
Under the circumstances as described above, it has been proposed to provide hydraulic switches that operate in response to the hydraulic pressures of the respective friction devices that constitute the automatic transmission, and determine the presence of a fault of any of the solenoid valves by monitoring the outputs of the hydraulic switches. Such diagnostic systems are disclosed in, for example, JP-A-2001-116134 and JP-A-2006-77892. In these systems, the hydraulic switch provided for each of the friction devices generates an “ON” signal when the hydraulic pressure applied to the corresponding friction device is increased and the friction device is engaged, and generates an “OFF” signal when the hydraulic pressure applied to the corresponding friction device is reduced and the friction device is released. If, upon shifting of the automatic transmission to a target gear position during operation of the vehicle, the “OFF” signal is generated from the hydraulic switch associated with the friction device that is supposed to be engaged for shifting, for example, it is determined that the solenoid valve that controls the hydraulic pressure of the friction device in question is at fault. Thus, even in the event that a fault, such as disconnection or a short-circuit, occurs to a solenoid of any of the solenoid valves, the diagnostic system as described above makes it possible to perform a fail-safe process suitable for the fault condition, and thus prevent locking, or other malfunctions, of the automatic transmission in advance.
As discussed above, it is certainly possible to make fault diagnoses of the solenoid valves by monitoring the outputs of the hydraulic switches which correspond with the respective friction devices of the automatic transmission and operate in response to the hydraulic pressures of the friction devices. It is, however, to be noted that the hydraulic switches are adapted to generate the “ON” or “OFF” signals depending upon the operating (engaged/released) states of the corresponding friction devices, or changes in the states of the devices. Therefore, the frequency of diagnosing the solenoid valves inevitably varies from one solenoid valve to another, depending upon the manner of operating the automatic transmission itself, even though fault diagnoses may be performed on the solenoid valves in the above manner. Thus, if the transmission is less likely to be shifted to a certain gear position due to, for example, the driving practice of the driver, the frequency of diagnoses is naturally reduced with respect to each solenoid valve for regulating the hydraulic pressure applied to each friction device that is to be engaged for establishing the gear position. Thus, the diagnostic system still suffers a problem in terms of the flexibility or freedom with which fault diagnoses are performed.